The Barrier remains a concern

In the wake of the Meager landslide is the region prepared for another natural catastrophe?

The Meager area isn't the only part of Sea to Sky that's vulnerable to landslides.

The second biggest landslide in Canadian history, which fell about 60 kilometres from Pemberton last month, creating a natural dam in Meager Creek and closing the Lillooet Forest Service Road, has raised concerns in the region about the risks that landslides post to various communities.

Some concerns persist in particular about The Barrier, a natural lava dam located about halfway between Whistler and Squamish in Garibaldi Provincial Park. It's about 300 metres thick and holding back Lesser Garibaldi Lake.

Though the lake is about two kilometres from The Barrier, there are nevertheless worries that the dam could collapse, releasing the water and causing downstream damage as far as the District of Squamish and the waters of Howe Sound.

The story of The Barrier goes back about 13,000 years to when Mount Price, one of the 13 cones in the Garibaldi volcano, erupted and sent a massive lava flow down the Rubble Creek valley.

At the time the valley was still filled with the Cordilleran ice sheet, a major sheet that covered a large part of North America including Western Montana, northern Washington and just about all of British Columbia.

The lava flow stopped at the edge of the glacier that filled the Cheakamus Valley. When the lava met the glacier it retreated, creating a crevasse-like opening against the ice that was later filled with more lava. This process took place several times and created vertical slabs of lava right across the width of the Rubble Creek valley.

The process created a natural lava dam that blocked the valley and allowed Garibaldi Lake to form behind it.

"This was identified by one of my geology profs, Bill Matthews, years and years ago," volcanologist Jack Souther said in an interview. "He noted that the columns at the toe of The Barrier were horizontal. They'd been quenched against the ice and therefore had grown back at right angles to this vertical surface."

Once the glacier melted, it left behind a vertical cliff of lava now known as The Barrier.

As geologist Frank Baumann tells it, the lava flows that make up The Barrier are brittle and broken, creating constant rockfalls that vary in size. The falling material builds up at the base of the Barrier Cliff and creates a "big sloping apron" of material known as talus.

In an e-mailed response, Baumann described two theories that exist regarding future landslides at The Barrier.

The late UBC Professor William Mathews advanced the first one. He said big vertical slabs of lava that make up the Barrier Cliff occasionally collapse and form massive rock avalanches that move down the valley.